ii Routing Protocols and Concepts, CCNA Exploration Labs and Study Guide. Routing Protocols and Use the Lab PDF for more details. 46 Routing Protocols. 1. Chapter 1: Routing. Concepts. Routing and Switching Essentials v Explain how a router builds a routing table using a dynamic routing protocol. structure and messaging. ⇨give an high-level overview of IP routing protocols . a key concept for The Internet each AS selects its own routing protocol to.
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Cisco Systems, Inc. All rights reserved. Cisco Confidential. Presentation_ID. 1. Chapter 4: Routing. Concepts. Routing Protocols. Routing Protocols and homeranking.info Created by Deonte R. Carroll on Sep 19, AM. Last modified by Deonte R. Carroll on Sep 19, AM. ii Routing Protocols and Concepts, CCNA Exploration Companion Guide Using a Networker's Journal” PDF booklet providing important insight into the value.
The interface must also be connected to another device a hub, a switch, another router, etc. There is still one more command that we need to enter, the clock rate command, on the router with the DCE cable. For static routes with outbound point-to-point serial networks, it is best to configure static routes with only the exit interface. Static Routing 47 the network address as indicated by the prefix of the route. Reading the routing tables will soon be- come second nature. When a router only has its interfaces configured, and the routing table contains the directly connected networks but no other routes, only devices on those directly connected networks are reachable.
Asymmetric routing is more common in the Internet, which uses the BGP routing protocol than it is in most internal networks. This example implies that when designing and troubleshooting a network, the network administra- tor should check the following routing information: Asymmetrical routing is not uncommon, but sometimes can pose additional issues. Refer to Packet Use the Packet Tracer Activity to investigate a fully-converged network with connected, static, and Tracer Activity for this chapter dynamic routing.
Before sending a packet out the proper exit interface, the IP packet needs to be encapsulated into a Layer 2 data link frame. Later in this section we will follow an IP packet from source to destination, examining the encapsulation and decapsulation process at each router.
The IP packet header has specific fields that contain information about the packet and about the sending and receiving hosts. Below is a list of the fields in the IP header and a brief description for each one. The other fields are important but are outside the scope of this course. The data link source address is the Layer 2 address of the interface that sent the data link frame. The data link destination address is the Layer 2 address of the interface of the destination device. Both the source and destination data link interfaces are on the same network.
As a packet is forwarded from router to router, the Layer 3 source and destination IP addresses will not change; however, the Layer 2 source and des- tination data link addresses will change.
This process will be examined more closely later in this section. Routing with NAT is discussed in a later course. The Layer 3 IP packet is encapsulated in the Layer 2 data link frame associated with that interface. In this example, we will show the Layer 2 Ethernet frame. The figure shows the two compatible versions of Ethernet. Below is a list of the fields in an Ethernet frame and a brief description of each one.
Whenever multiple paths to reach the same network exist, each path uses a different exit interface on the router to reach that network. The best path is selected by a routing protocol based on the value or metric it uses to determine the distance to reach a network. Some routing protocols, such as RIP, use simple hop-count, which the number of routers between a router and the destination network.
Other rout- ing protocols, such as OSPF, determine the shortest path by examining the bandwidth of the links, and using the links with the fastest bandwidth from a router to the destination network. Dynamic routing protocols typically use their own rules and metrics to build and update routing ta- bles.
A metric is the quantitative value used to measure the distance to a given route. For example, a router will prefer a path that is 5 hops away over a path that is 10 hops away.
The primary objective of the routing protocol is to determine the best paths for each route to in- clude in the routing table. The routing algorithm generates a value, or a metric, for each path through the network.
Metrics can be based on either a single characteristic or several characteris- tics of a path. Some routing protocols can base route selection on multiple metrics, combining them into a single metric.
The smaller the value of the metric, the better the path. Comparing Hop Count and Bandwidth Metrics Two metrics that are used by some dynamic routing protocols are: Each router is equal to one hop. A hop count of four indicates that a packet must pass through four routers to reach its destination.
If multiple paths are available to a destination, the routing protocol, such as RIP, picks the path with the least number of hops. The best path to a network is determined by the path with an accumulation of links that have the highest bandwidth values, or the fastest links.
Speed is technically not an accurate description of bandwidth because all bits travel at the same speed over the same physical medium. Bandwidth is more accurately defined as the number of bits that can be transmitted over a link per second. When hop count is used as the metric, the resulting path may sometimes be suboptimal. For exam- ple, consider the network shown in the figure.
If RIP is the routing protocol used by the three routers, then R1 will choose the suboptimal route through R3 to reach PC2 because this path has fewer hops. Bandwidth is not considered. Packets will be able to reach their destination sooner using the two, faster T1 links as compared to the single, slower 56 Kbps link. When a router has multiple paths to a destination network and the value of that metric hop count, bandwidth, etc.
The routing table will contain the single destination network but will have multiple exit interfaces, one for each equal cost path. The router will forward packets using the multiple exit interfaces listed in the routing table. If configured correctly, load balancing can increase the effectiveness and performance of the net- work.
Equal cost load balancing can be configured to use both dynamic routing protocols and static routes. Equal Cost Paths and Unequal Cost Paths Just in case you are wondering, a router can send packets over multiple networks even when the metric is not the same if it is using a routing protocol that has this capability.
Introduction to Routing and Packet Forwarding 27 unequal cost load balancing. Refer to Packet Use the Packet Tracer Activity to explore a routing table that is using equal cost load balancing. One of three path determinations results from this search: Remote Network - If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
No Route Determined - If the destination IP address of the packet does not belong to either a con- nected or remote network, and if the router does not have a default route, then the packet is dis- carded. In the first two results, the router re-encapsulates the IP packet into the Layer 2 data link frame format of the exit interface. The type of Layer 2 encapsulation is determined by the type of inter- face. For example, if the exit interface is FastEthernet, the packet is encapsulated in an Ethernet frame.
The following section demonstrates this process. The switching function is the process used by a router to accept a packet on one interface and for- ward it out another interface.
A key responsibility of the switching function is to encapsulate pack- ets in the appropriate data link frame type for the outgoing data link. What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps: Decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer. Examines the destination IP address of the IP packet to find the best path in the routing table.
Encapsulates Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit inter- face. Click Play to view the animation. If the resulting TTL value is zero, the router discards the packet.
The TTL is used to prevent IP packets from traveling endlessly over networks due to a routing loop or other misfunction in the network. Routing loops are discussed in a later a chapter. As the IP packet is decapsulated from one Layer 2 frame and encapsulated into a new Layer 2 frame, the data link destination address and source address will change as the packet is forwarded from one router to the next.
The Layer 2 data link source address represents the Layer 2 address of the outbound interface. The Layer 2 destination address represents the Layer 2 address of the next- hop router.
If the next hop is the final destination device, it will be the Layer 2 address of that device. It is very likely that the packet will be encapsulated in a different type of Layer 2 frame than the one in which it was received. For example, the packet might be received by the router on a FastEthernet interface, encapsulated in an Ethernet frame, and forwarded out a serial interface en- capsulated in a PPP frame.
Remember, as a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. However, the Layer 2 data link addresses change at every hop as the packet is decapsulated and re-encapsulated in a new frame by each router.
Path Determination and Switching Function Details Can you describe the exact details of what happens to a packet at Layer 2 and Layer 3 as it travels from source to destination? If not, study the animation and follow along with the discussion until you can describe the process on your own.
Step 1: PC1 knows the network it belongs to by doing an AND operation on its own IP address and subnet mask, which results in its network address.
If the result is the same as its own network, PC1 knows that the destination IP address is on its own network and it does not need to forward the packet to the default gateway, the router. If the AND operation results in a different network ad- dress, PC1 knows that the destination IP address is not on its own network and that it must forward this packet to the default gateway, the router.
If an AND operation with the destination IP address of the packet and the subnet mask of PC1 results in a different network address than what PC1 has determined to be its own network address, this address does not necessarily reflect the actual remote network address. PC1 only knows that if the destination IP address is on its own network, the masks will be the same and the network addresses would be the same.
The mask of the remote network might be a different mask. If the destination IP address results in a different network address, PC1 will not know the actual remote network address - it only knows that it is not on its own network. What if this entry does not exist in the ARP table? Step 2: Router R1 receives the Ethernet frame 1. R1 will therefore copy the frame into its buffer. R1 sees that the Ethernet Type field is 0x, which means that the Ethernet frame contains an IP packet in the data portion of the frame.
R1 decapsulates the Ethernet frame. In this example, the routing table has a route for the The destination IP address of the packet is R1 looks up the next-hop IP address of R2 sends back an ARP reply. R1 then updates its ARP cache with an entry for Step 3: Packet arrives at router R2 Click Play to view the animation.
R2 sees that the Ethernet Type field is 0x, which means that the Ethernet frame contains an IP packet in the data portion of the frame. R2 decapsulates the Ethernet frame. Because the exit interface is not an Ethernet net- work, R2 does not have to resolve the next-hop-IP address with a destination MAC address.
In this case, the Layer 2 encapsulation is PPP; therefore, the data link destination address is set to a broadcast. Remem- ber, there are no MAC addresses on serial interfaces.
Step 4: The packet arrives at R3 1. R3 receives and copies the data link PPP frame into its buffer. R3 decapsulates the data link PPP frame. R3 searches the routing table for the destination IP address of the packet. This means that the packet can be sent directly to the destination device and does not need to be sent to another router. Because the exit interface is a directly connected Ethernet network, R3 needs to resolve the desti- nation IP address of the packet with a destination MAC address.
R3 updates its ARP cache with an entry for Step 5: PC2 will therefore copy the rest of the frame into its buffer. PC2 sees that the Ethernet Type field is 0x, which means that the Ethernet frame contains an IP packet in the data portion of the frame. Summary We have just examined the encapsulation and decapsulation process of a packet as it is forwarded from router to router, from the originating source device the final destination device. We have also been introduced to the routing table lookup process, which will be discussed more thoroughly in a later chapter.
We have seen that routers are not involved only in Layer 3 routing decisions, but that they also participate in Layer 2 processes, including encapsulation, and on Ethernet networks, ARP. Routers also participate in Layer 1, which is used to transmit and receive the data bits over the physical medium. Routing tables contain both directly connected networks and remote networks. It is because routers contain addresses for remote networks in their routing tables that routers know where to send packets destined other networks, including the Internet.
In the following chapters will learn how the routers build and maintain these routing tables - either by the use of manually entered static routes or through the use of dynamic routing protocols.
If you are comfortable with these skills, you can substi- tute Lab 1. Remember, however, that Tracer Activity Packet Tracer is not a substitute for a hands-on lab experience with real equipment. Use the Lab PDF for more details. If you need a review of these skills, you can substitute Lab 1. Remember, however, that Tracer Activity for this chapter Packet Tracer is not a substitute for a hands-on lab experience with real equipment.
A summary of the instructions is provided within the activity. Given an address space and network Lab Activity for this chapter requirements, you are expected to design and implement an addressing scheme in a two-router topology.
The main purpose of a router is to connect multiple networks and forward packets from one net- work to the next. This means that a router typically has multiple interfaces. Each interface is a member or host on a different IP network. The router has a routing table, which is a list of networks known by the router.
The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router. Remote networks are added to the routing table in two ways: Static routes do not have as much overhead as dynamic routing protocols; however, static routes can require more maintenance if the topology is constantly changing or is unstable.
Dynamic routing protocols automatically adjust to changes without any intervention from the net- work administrator. Dynamic routing protocols require more CPU processing and also use a cer- tain amount of link capacity for routing updates and messages. In many cases, a routing table will contain both static and dynamic routes. Routers make their primary forwarding decision at Layer 3, the Network layer.
However, router in- terfaces participate in Layers 1, 2, and 3. Layer 3 IP packets are encapsulated into a Layer 2 data link frame and encoded into bits at Layer 1. Router interfaces participate in Layer 2 processes as- sociated with their encapsulation. In the next chapter, we will examine the configuration of static routes and introduce the IP rout- ing table.
The Packet Tracer Skills Integration Challenge Activity for this chapter integrates all the knowl- edge and skills you acquired in previous courses and the first chapter of this course. In this activ- Refer to Packet ity, you build a network from the ground up.
Starting with an addressing space and network Tracer Activity requirements, you must implement a network design that satisfies the specifications. On one LAN add a client host, and on the other end add a web server. On each LAN include a switch be- tween the computer and the router.
Assume that each router has a route to each of the LANs, simi- lar to that in 1. What happens when the host requests a web page from the web server? Look at all of the processes and protocols involved starting with the user entering a URL such as www.
This includes protocols learned in Exploration 1 as well as information learned in this chapter. Introduction to Routing and Packet Forwarding 33 See if you can determine each of the processes that happen starting with the client needing to re- solve www.
What are all of the protocols and processes involved starting with the DNS request to getting the first packet with http information from the web server. Is the first packet the web server receives from the client the request for the web page?
How do they update their MAC address tables and how do they determine how to forward the frame? Chapter Quiz Take the chapter quiz to test your knowledge.
Routers are the devices responsible for the transfer of packets from one net- work to the next. As we learned in the previous chapter, routers learn about remote networks either dynamically using routing protocols or manually using static routes.
In many cases routers use a combination of both dynamic routing protocols and static routes. This chapter focuses on static routing. Static routes are very common and do not require the same amount of processing and overhead as we will see with dynamic routing protocols.
In this chapter, we will follow a sample topology as we configure static routes and learn trou- bleshooting techniques. In the process, we will examine several key IOS commands and the results they display. We will also introduce the routing table using both directly connected networks and static routes. As you work through the Packet Tracer activities associated with these commands, take the time to experiment with the commands and examine the results.
Reading the routing tables will soon be- come second nature. Routers are primarily responsible for interconnecting networks by: The router is the junction or intersection that connects multiple IP networks. The routers primary forwarding decision is based on Layer 3 information, the destination IP address. The routing table will ultimately determine the exit inter- face to forward the packet and the router will encapsulate that packet in the appropriated data link frame for that outgoing interface.
The topology consists of three routers, labeled R1, R2, and R3. Each router in this example is a Cisco A Cisco router has the following interfaces: In addition, Packet Tracer activities are available throughout the discussion of static routing so that you can practice skills as they are presented. Lab 2. As you can see in the figure, Cisco routers support many different connector types. Serial Connectors Click 1 in the figure.
Memorizing these connection types is not important. Just know that a router has a DB port that can support five different cabling stan- dards. Because five different cable types are supported with this port, the port is sometimes called a five-in-one serial port. The other end of the serial cable is fitted with a connector that is appropri- ate to one of the five possible standards.
The documentation for the device to which you want to connect should indicate the standard for that device. Click 2 and 3 in the figure. Newer routers support the smart serial interface that allows for more data to be forwarded across fewer cable pins. The serial end of the smart serial cable is a pin connector.
It is much smaller than the DB connector used to connect to a five-in-one serial port. These transition cables sup- port the same five serial standards and are available in either DTE or DCE configurations. In a production setting, the cable type is determined for you by the WAN service you are using. Ethernet Connectors Click 4 in the figure. Chapter 2: At each end of an RJ cable, you should be able to see eight colored strips, or pins. An Ethernet cable uses pins 1, 2, 3, and 6 for transmitting and receiving data.
Two types of cables can be used with Ethernet LAN interfaces: Wireless connectivity is discussed in another course. Refer to Packet Use the Packet Tracer Activity to build the topology that you will use for the rest of this chapter. Tracer Activity for this chapter You will add all the necessary devices and connect them with the correct cabling.
Ini- tially, the routing table is empty if no interfaces have been configured.
As you can see in the routing table for R1, no interfaces have been configured with an IP address and subnet mask. Static routes and dynamic routes will not be added to the routing table until the appropriate local interfaces, also known as the exit interfaces, have been configured on the router.
This proce- dure will be examined more closely in later chapters. Interfaces and their Status The status of each interface can be examined by using several commands. The show interfaces command shows the status and gives a detailed description for all interfaces on the router. As you can see, the output from the command can be rather lengthy.
For example: Administra- tively down means that the interface is currently in the shutdown mode, or turned off. Line proto- col is down means, in this case, that the interface is not receiving a carrier signal from a switch or the hub. This condition may also be due to the fact that the interface is in shutdown mode. The reason for this is because we have not yet configured IP addresses on any of the interfaces.
The show ip interface brief command can be used to see a portion of the interface informa- tion in a condensed format. Click show running-config in the figure.
The show running-config command displays the current configuration file that the router is using. Configuration commands are temporarily stored in the running configuration file and imple- mented immediately by the router. Use the show ip interface brief command to quickly verify that interfaces are up and up administratively up and line protocol is up.
By default, all router interfaces are shutdown, or turned off. To enable this interface, use the no shutdown command, which changes the interface from administratively down to up. Static Routing 39 changed state to up Both of these messages are important. The first changed state to up message indicates that, physi- cally, the connection is good. If you do not get this first message, be sure that the interface is prop- erly connected to a switch or a hub.
Although enabled with no shutdown, an Ethernet interface will not be active, or up, unless it is receiving a carrier signal from another device switch, hub, PC, or another router.
The second changed state to up message indicates that the Data Link layer is operational. However, WAN interfaces in a lab environment require clocking on one side of the link as discussed in Lab 1.
The IOS often sends unsolicited messages similar to the changed state to up messages just dis- cussed. As you can see in the figure, sometimes these messages will occur when you are in the middle of typing a command, such as configuring a description for the interface. The IOS message does not affect the command, but it can cause you to lose your place when typing. Click Logging Synchronous in the figure. In order to keep the unsolicited output separate from your input, enter line configuration mode for the consoled port and add the logging synchronous command, as shown.
You will see that mes- sages returned by IOS no longer interfere with your typing. Reading the Routing Table Now look at routing table shown in the figure. The interface was configured with the Examine the following line of output from the table: C In other words, R1 has an interface that belongs to this network. The meaning of C is defined in the list of codes at the top of the routing table. The Having a single route represent an entire network of host IP addresses makes the routing table smaller, with fewer routes, which results in faster routing table lookups.
The routing table could contain all individual host IP addresses for the A phone book is a good analogy for a routing table structure. A phone book is a list of names and phone numbers, sorted in alphabetical order by last name. When looking for a number, we can as- sume that the fewer names there are in the book, the faster it will be to find a particular name.
The phone book only contains one listing for each phone number. For example, the Stanford fam- ily might be listed as: Stanford, Harold, Evergreen Terrace, This is the single entry for everyone who lives at this address and has the same phone number.
The phone book could contain a listing for every individual, but this would increase the size of the phone book.
For example, there could be a separate listing for Harold Stanford, Margaret Stanford, Brad Stanford, Leslie Stanford, and Maggie Stanford - all with the same address and phone num- ber. If this were done for every family, the phone book would be larger and take longer to search. Routing tables work the same way: The fewer the entries in the routing table, the faster the lookup process.
To keep routing tables smaller, network addresses with subnet masks are listed instead of individual host IP addresses. The topic of host routes is discussed in another course. The no shutdown command changed the interface from administratively down to up. Notice that the IP address is now displayed. Click show ip interface brief in the figure.
The show ip interface brief command also shows verifies this same information. The show running-config command shows the current configuration of this interface. When the interface is disabled, the running-config command displays shutdown; however, when the inter- face is enabled, no shutdown is not displayed. Each interface must belong to a separate subnet. The IOS will return the following error message if you attempt to configure the second interface with the same IP subnet as the first interface: For example, PC1 would be configured with a host IP address belonging to the This means that these interfaces have a Layer 2 MAC address, as shown in the figure.
The show interfaces command displays the MAC address for the Ethernet interfaces. If a router has a packet destined for a device on a directly connected Eth- ernet network, it checks the ARP table for an entry with that destination IP address in order to map it to the MAC address.
The Ethernet frame, with the en- capsulated packet, is then sent via that Ethernet interface. Follow the additional Tracer Activity for this chapter instructions provided in the activity to examine the ARP process in simulation mode. This interface is on the This will be discussed in more detail in a later course. In this course, we will be using dedicated, serial point-to-point connections between two routers. The serial interface will be in the up state only after the other end of the serial link has also been properly configured.
As you can see, the link is still down. The link is down because we have not yet configured and en- abled the other end of the serial link. However, because both interfaces are members of the same network, they both must have IP addresses that belong to the However, the line protocol is still down. This is because the interface is not receiving a clock signal. There is still one more command that we need to enter, the clock rate command, on the router with the DCE cable. The clock rate command will set the clock signal for the link.
Configuring the clock signal will be discussed in the next section. Serial interfaces require a clock signal to control the timing of the communications.
By default, Cisco routers are DTE devices. Roll over the cables and devices in the figure to see what they are.
Configuring Serial Links in a Lab Environment For serial links that are directly interconnected, as in a lab environment, one side of a connection must be considered a DCE and provide a clocking signal. To configure a router to be the DCE device: Connect the DCE end of the cable to the serial interface. Configure the clock signal on the serial interface using the clock rate command.
The serial cables used in the lab are typically one of two types. If a cable is connected between the two routers, you can use the show controllers command to determine which end of the cable is attached to that interface. The available clock rates, in bits per second, are , , , , , , , , , , , , , , , and Some bit rates might not be available on certain serial interfaces. Verifying the Serial Interface Configuration As you can see from the figure, we can determine that the line protocol is now up and verify this on both ends of the serial link by using the show interfaces and show ip interface brief com- mands.
Remember, the serial interface will be up only if both ends of the link are configured cor- rectly. In our lab environment, we have configured the clock rate on the end with the DCE cable. R1 ping If we issue the show ip route command on R1, we will see the directly connected route for the R1 show running-config Note: Although the clock rate command is two words, the IOS spells clockrate as a single word in the running configuration and startup configuration files.
A routing table is a data structure used to store routing information acquired from different sources. The main purpose of a routing table is to provide the router with paths to different destination networks. R1 and R2 only have routes for directly connected networks. Observing Routes as They are Added to the Routing Table We will now take a closer look at how directly connected routes are added to, and deleted from, the routing table.
In contrast to show commands, debug commands can be used to monitor router operations in real time. The debug ip routing command will let us see any changes that the router performs when adding or removing routes. We will configure the interfaces on the R2 router and examine this process.
First, we will enable debugging with the debug ip routing command so that we can see the di- rectly connected networks as they are added to the routing table. Because the FastEthernet interface connects to the The routing table now shows the route for the directly connected network The debug ip routing command displays routing table processes for any route, whether that route is a directly connected network, a static route, or a dynamic route.
Click Disable Debug in the figure. Disable debug ip routing by using either the undebug ip routing command or the undebug all command. This change will overwrite the previous entry. There are ways to configure a single interface with multiple IP addresses, as long as each address is on a different subnet. This topic will be discussed in a later course. To remove a directly connected network from a router, use these two commands: The shutdown command is used to disable interfaces.
The IP address, however, will still be in the configuration file, running-config. After the shutdown command is used, you can remove the IP address and subnet mask from the in- terface. The order in which you perform these two commands does not matter.
Click Debug 2 in the figure. R2 config-if no ip address Disable debugging: R2 undebug all All possible debugging has been turned off Click Routing Table 2 in the figure.
To verify that the route was removed from the routing table, we use the command show ip route. Notice that the route to Reconfiguring the interface to continue with the chapter. To reconfigure the interface, simply enter the commands again: Debug commands, especially the debug all command, should be used sparingly.
These commands can disrupt router operations. Debug commands are useful when configuring or troubleshooting a network; however, they can make intensive use of CPU and memory resources. Debug commands should be used with caution on production net- works because they can affect the performance of the device. You will also use debug ip Tracer Activity for this chapter routing to observe the routing table processes. The figure shows the rest of the configurations for routers R2 and R3.
Click show ip route in the figure. By reviewing the routing tables in the figure, we can verify that all directly connected networks are installed for routing. Regardless of what routing scheme you ultimately config- ure - static, dynamic, or a combination of both - verify your initial network configurations with the show ip interface brief command and the show ip route command before proceeding with more complex configurations.
When a router only has its interfaces configured, and the routing table contains the directly connected networks but no other routes, only devices on those directly connected networks are reachable.
Because these routers only know about their directly connected networks, the routers can only communicate with those devices on their own directly connected LANs and serial networks.
For example, PC1 in the topology has been configured with the IP address PC1 has also been configured with the default gateway IP address Because R1 only knows about directly connected networks, it can forward packets from PC1 to devices on the R2 only knows about its three directly connected networks.
Try to predict what will happen if we ping one of the FastEthernet interfaces on one of the other routers. Click ping in the figure. Notice that the pings failed, as indicated by the series of five periods.
It failed because R2 does not have a route in its routing table that matches either Static Routing 47 the network address as indicated by the prefix of the route. Play the first animation in the figure. If you convert these addresses to binary and compare them, as shown in the animation, you will see that the first 24 bits of this route do not match because the 23rd bit does not match.
Therefore, this route is rejected. Therefore, this route is also rejected, and the process moves on to the next route in the routing table.
As shown, 10 of the first 24 bits do not match. Because there are no more routes in the routing table, the pings are discarded. Click Pings are sent to R3 on the figure and play the animation. This time the ping succeeds! It is successful because R2 has a route in its routing table that matches The first two routes, But the last route, R2 is now done making the forwarding decisions for this packet; the decisions made by other routers regarding this packet are not its concern.
A Closer Look. Tracer Activity for this chapter 2. CDP is an information-gathering tool used by network administrators to get information about directly connected Cisco devices. CDP is a proprietary tool that enables you to access a summary of proto- col and address information about Cisco devices that are directly connected.
By default, each Cisco device sends periodic messages, which are known as CDP advertisements, to directly con- nected Cisco devices. Most network devices, by definition, do not work in isolation. A Cisco device frequently has other Cisco devices as neighbors on the network.
Information gathered from other devices can assist you in making network design decisions, troubleshooting, and making changes to equipment.
CDP can be used as a network discovery tool, helping you to build a logical topology of a network when such documentation is missing or lacking in detail. Familiarity with the general concept of neighbors is important for understanding CDP as well as for future discussions about dynamic routing protocols.
Layer 3 Neighbors At this point in our topology configuration, we only have directly connected neighbors. At Layer 3, routing protocols consider neighbors to be devices that share the same network address space. For example, R1 and R2 are neighbors. Both are members of the R2 and R3 are also neighbors because they both share the But R1 and R3 are not neighbors because they do not share any network address space.
If we connected R1 and R3 with a cable and configured each with an IP address from the same network, then they would be neighbors. Therefore, CDP neighbors are Cisco devices that are directly con- nected physically and share the same data link. Assuming that all routers and switches in the figure are Cisco devices running CDP, what neigh- bors would R1 have? Can you determine the CDP neighbors for each device? Click the Topology button in the figure. In our chapter topology, we can see the following CDP neighbor relationships: Notice the difference between Layer 2 and Layer 3 neighbors.
The switches are not neighbors to the routers at Layer 3, because the switches are operating at Layer 2 only. However, the switches are Layer 2 neighbors to their directly connected routers. CDP Operation Examine the output from the show cdp neighbors and show cdp neighbors detail commands in the figure.
Notice that R3 has gathered some detailed information about R2 and the switch con- nected to the Fast Ethernet interface on R3. CDP automatically discovers neighbor- ing Cisco devices running CDP, regardless of which protocol or suites are running.
Tracer Activity for this chapter Practice enabling and disabling CDP - globally and on a per-interface basis.
Investigate the power of using CDP to discover the topology of a network. For each CDP neighbor, the following information is displayed: The show cdp neighbors detail command also reveals the IP address of a neighboring device. This command is very helpful when two Cisco routers cannot route across their shared data link. The show cdp neighbors detail command will help determine if one of the CDP neighbors has an IP configuration error.
For network discovery situations, knowing the IP address of the CDP neighbor is often all the in- formation needed to telnet into that device. In this fashion, you can telnet around a network and build a logical topology. In the next Packet Tracer Activity, you will do just that. Yes, it could be. You may already have seen CDP packets in your packet capturing labs from a previous course.
Click Disabling CDP in the figure. Router config-if no cdp enable Refer to Packet CDP show commands can be used to discover information about unknown devices in a network. Tracer Activity for this chapter CDP show commands display information about directly connected Cisco devices, including an IP address that can be used to reach the device. You can then telnet to the device and repeat the process until the entire network is mapped.
Dynamic routing protocols are intro- duced in the next chapter. Static routes Static routes are commonly used when routing from a network to a stub network.
A stub network is a network accessed by a single route. For an example, see the figure. Here we see that any net- work attached to R1 would only have one way to reach other destinations, whether to networks at- tached to R2 or to destinations beyond R2. Therefore, network Running a routing protocol between R1 and R2 is a waste of resources because R1 has only one way out for sending non-local traffic.
Therefore, static routes are configured for connectivity to re- mote networks that are not directly connected to a router. We will also see how to configure a de- fault static route from R1 to R2 later in the chapter so that R1 can send traffic to any destination beyond R2. The ip route command The command for configuring a static route is ip route. The complete syntax for configuring a static route is: As shown in the figure, we will use a simpler version of the syntax: The subnet mask can be modified to summarize a group of networks.
One or both of the following parameters must also be used: However, the ip-ad- dress parameter could be any IP address, as long as it is resolvable in the routing table. These are the routes currently in its routing table. The remote networks that R1 does not know about are: First, enable debug ip routing to have the IOS display a message when the new route is added to the routing table.
Then, use the ip route command to configure static routes on R1 for each of these networks. The figure shows the first route configured. R1 debug ip routing R1 conf t R1 config ip route In other words, the next-hop IP address Verifying the Static Route The output from debug ip routing shows that this route has been added to the routing table. The static route entry is highlighted. Configuring Routes to Two More Remote Networks The commands to configure the routes for the other two remote networks are shown in the figure.
Notice that all three static routes configured on R1 have the same next-hop IP address: Using the topology diagram as a reference, we can see that this is true because packets for all of the remote networks must be forwarded to router R2, the next-hop router.
Use the show ip route command again to examine the new static routes in the routing table, as shown. S For now, this difference is not important. The static routes that have been configured can also be verified by examining the running configu- ration with the show running-config command. R1 copy running-config startup-config 2. Will packets from all these networks destined for network Principle 1: R1 does not consult the routing tables in any other routers.
Nor does it know whether or not those routers have routes to other networks. Making each router aware of remote networks is the responsibility of the network administrator.
Static Routing 53 Principle 2: For example, R1 has a route to the Any packets that match this route belong to the R1 does not know whether or not R2 has a route to the Again, the network administrator would be re- sponsible for ensuring that the next-hop router also has a route to this network.
Using Principle 2, we still need to configure the proper routing on the other routers R2 and R3 to make sure that they have routes to these three networks. Principle 3: This means that packets must travel in both directions between the end devices involved. A packet from PC1 may reach PC3 because all the routers involved have routes to the destination network Using Principle 3 as guidance, we will configure proper static routes on the other routers to make sure they have routes back to the Applying the Principles With these principles in mind, how would you answer the questions we posed regarding packets that originate from PC1?
Would packets from PC1 reach their destination? In this case, packets destined for This is because router R1 has a route to these networks through R2. When packets reach router R2, these networks are directly connected on R2 and are routed using its routing table. Packets destined for R1 has a static route to this network through R2. However, when R2 receives a packet, it will drop it because R2 does not yet contain a route for this network in its routing table.
Does this mean that any packets from these networks destined for If R2 or R3 receives a packet destined for Click R2 and R3 Static Routes in the figure. With the commands shown in the figure, all routers now have routes to all remote networks. Examine the routing tables in the figure to verify that all routers now have routes to all remote networks. Connectivity can be further verified by pinging remote router interfaces from router R1, as shown in the figure.
Full connectivity is now achieved for the devices in our topology. This is known as route resolvability. R1 has a static route for the remote network R1 must determine how to reach the next-hop IP address It will do a second search looking for a match for In this case, the IP address This lookup tells the routing table process that this packet will be forwarded out that interface.
There- fore, it actually takes two routing table lookup processes to forward any packet to the When the router has to perform multiple lookups in the routing table be- fore forwarding a packet, it is performing a process known as a recursive lookup. In this example: The next-hop IP address of the static route, Every route that references only a next-hop IP address, and does not reference an exit-interface, must have the next-hop IP address resolved using another route in the routing table that has an exit interface.
Typically, these routes are resolved to routes in the routing table that are directly connected net- works, because these entries will always contain an exit interface. We will see in the next section that static routes can be configured with an exit interface. This means that they do not need to be resolve using another route entry. If the interface comes back up is enabled again with no shutdown , the IOS routing table process will reinstall these static routes back into the routing table.
Static Routing 55 2. In the run- ning configuration, note the following line: However, most static routes can be configured with an exit interface, which allows the routing table to resolve the exit interface in a single search instead of two searches. The first thing to do is to delete the current static route. This is done using the no ip route command as shown in the figure. Notice that the entry in the routing table no longer refers to the next-hop IP address but refers directly to the exit interface.
This exit interface is the same one that the static route was resolved to when it used the next-hop IP address. The static route displays the route as directly connected. It is important to understand that this does not mean that this route is a directly connected network or directly connected route. This route is still a static route.
We will examine the importance of this fact when we discuss Adminis- trative Distances in the next chapter. Static routes and point-to-point networks Static routes that are configured with exit interfaces instead of next-hop IP addresses are ideal for most serial point-to-point networks. These types of point-to-point serial links are like pipes.
A pipe has only two ends. What enters one end can only have a single destination - the other end of the pipe. Under certain conditions, the network administrator will not want to configure the static route with an exit interface but with the next-hop IP address. This type of situation is beyond the scope of this course but is important to note. There is no way to modify an existing static route. The static route must be deleted and a new one configured.
To delete a static route, add no in front of the ip route command, followed by the rest of the static route to be removed. In the previous section, we had a static route: We configured a new static route using the exit interface: Skip to main content. Log In Sign Up. Nelson Opembe. Stores routing table. Information includes: Describe purpose of interface. Issue no shutdown command. Example of routing protocols include: They include: Basic configuration consists of: Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination.
The old static route must be deleted by placing no in front of the ip route o Example: Delete the current static route o Step 2: Configure the summary static route o Step 3: Stub routers that have a number of static routes all exiting the same interface are good candidates for a default route. Except that destination IP address and subnet mask are all zeros o Example: In the lab environment clock rates must be configured for DCE.
The type of cable used depends on what devices are being connected. Examples include: